Kirk, a longtime friend and former aide to Kennedy, is to be sworn in as senator this Friday. He will serve in the post until voters pick a permanent replacement during a special election next January.

The governor told reporters Thursday that matters before Congress and the nation are “too important for us to be one voice short” in Washington.

The appointment gives Democratic leaders in the Senate the 60th seat needed to circumvent efforts by Senate Republicans to prevent crucial votes on a health care reform bill. The late senator had called reform of the nation’s health care system the cause of his life.

Kirk said he was honoured to be appointed and added that he will not run in the special election. “He [Kennedy] often said that representing the people of Massachusetts in the Senate of the United States was the highest honour that he could possibly imagine, and it is certainly nothing that I imagined, but it will be my highest honour, as well,” Kirk said.

The appointment comes one day after the Massachusetts House and Senate passed legislation allowing the governor to name an interim replacement until a special election is held. Kennedy’s seat would have remained empty until the next elections, had that legislation not been approved. The rule change was one of Kennedy’s last wishes.